Superconducting RF resonators potentially can be combined into filters of very high performance at small volume, having, for instance, low insertion loss and sharp "skirts". However, it has been found that the power handling capacity of resonators that utilize a high temperature superconductor (HTS) material such as YBa.sub.2 Cu.sub.3 -oxide (conventionally referred to as "YBCO") frequently is limited, typically due to the presence of localized high current density in consequence of the Meissner effect. However, the advantage offered by use of a superconductor that does not have to be operated at liquid He temperature is so significant that there is a very strong incentive to improve the power handling capacity of resonators that use HTS material.
Recently a resonator geometry that can yield improved power handling capabilities was disclosed. The geometry is selected such that substantially no currents flow parallel to the edge of the HTS material. This is achieved with a disk resonator operating in the TM010 mode, wherein currents flow back and forth radially between the center and the edge of the disk. See S. Kolesov et al., Extended Abstract, International Superconducting Electronics Conference, MG3, pp. 272-274, June 1997, where a HTS TM010-mode disk resonator is disclosed, and Zhi-Yuan Shen et al., IEEE Transactions on Applied Superconductivity, Vol. 7(2), June 1997, pp. 2446-2453.
Generally two or more resonators are combined to provide a multipole filter. Kolesov et al., op. cit., disclose a 2-pole and a 4-pole filter, each comprising HTS TM010-mode resonators. The latter comprises two stacks of two disk resonators, with in-plane coupling elements providing the coupling between adjacent resonators (the second and third), and the cross coupling between the first and fourth resonators. A center hole is additionally used for the coupling between resonators.
Although the above discussed filters have relatively small size and light weight, and are said to be capable of handling up to 60 W of transmitted power, improvements would still be desirable. For instance, it would be desirable to have available a more compact filter design providing improved heat removal and relatively simple tuning. This application discloses such filters. The filters can, for instance, be used as transmit filters in base stations of a wireless communication system.